"obviously the shadow mesh is parented to the player, but it has to stay aligned also to a particular light source"
Which is why you DON'T parent the shadow mesh to the player. Just use commands like EntityX to move the shadow mesh along with the player every frame. And move the shadow after UpdateWorld to take collisions into account.
Which is why you DON'T parent the shadow mesh to the player. Just use commands like EntityX to move the shadow mesh along with the player every frame. And move the shadow after UpdateWorld to take collisions into account.